U.S. Pat. No. 3,778,114 granted Dec. 11, 1973 discloses a pneumatic outlet including an arcuate segment control valve located in the discharge tube. In closed position the control valve closes an opening between the lower inner edges of sloping outlet side walls. The control valve comprises an arcuate segment of approximately 270.degree., and includes contoured openings to control the application of vacuum suction to the lading being unloaded. The control valve is rotatable to a position within the tube to allow lading to enter the tube.
However, with hard plastic pellets such as acrylic resins, high density face cut polypropylene, and face cut polystyrene, a problem has developed in operation of the outlet. When the valve is rotated to the full open position with hard lading in the tube, and is then rotated in the same direction to the closed position, hard pellets may become trapped between the control valve and the tube wall. When this occurs the pellets tend to bend the control valve outward. When the control valve is bent in this manner the control valve tends to abut one or both lips located at the base of the hopper walls which define the opening into the discharge tube. While back and forth rotation of the control valve, rather than rotation in one direction, tends to minimize this problem, the problem still exists to some degree with back and forth rotation. Instructions to outlet attendants to use back and forth rotation are not always followed during commercial unloadings.
A related problem with the control valve in U.S. Pat. No. 3,778,114 is that the contour of openings in the control valve includes relatively deep valleys separated by flat land portions. The deep valleys tend to collect and trap pellets located between the valve and the seat or lip. These pellets are then moved with the control valve when it is further rotated to open position and are trapped between the valve and the tube wall. Such pellets then tend to cause bending of the control valve as described hereinabove.
It is also believed that the large arcuate segment of 270.degree. makes complete wash-out of the outlet more difficult. Furthermore, outlets because of their location on the bottom of hopper cars, are subjected to damage from foreign objects laying on the road bed. When the discharge tube is damaged on outlet covered in U.S. Pat. No. 3,778,114 the control valve often will not rotate, therefore the outlet cannot be opened and the car must be unloaded from the hatch opening.